The present invention relates to a photoconductive switch comprising a first layer of a first material and two contact electrodes arranged on the first layer and connectable to different potentials for applying a voltage through the first layer therebetween. The first layer is adapted to be conducting upon applying a voltage across the contact electrodes when a first side thereof is exposed to illumination through an illumination source of an energy high enough for creating free charge carriers in the first layer. The first layer has at least a part thereof doped with dopants providing substantially no free charge carriers for charge transport between the electrodes at the normal operation temperature of the switch.
A switch of this type finds many applications, and it may for instance be used in equipment for handling high electric power for switching high voltages (which may be 2-400 kV) and currents, for example in surge diverters, current limiters and the like. An advantage of a switch of this type is that illumination control provides for a very fast switching, which is of particular importance in high power applications for protection of the equipment when faults occur.
For a good function of such a switch it is important that the leakage current is low when the switch is open, i.e. when the first layer is not illuminated through the source, and that the switch may be quickly closed and have a low on-state voltage and a high current handling capability when the first layer is illuminated through the source. Accordingly, "dopants providing substantially no free charge carriers for charge transport between the electrodes at the normal operation temperature of the switch" means that the number of such free charge carriers will be low with respect to the number of the dopants at the operation temperature, which will normally, but not exclusively, be room temperature. The material of the first layer may be practically any layer that may be doped in this way, but the invention is in particular directed to wide band gap materials, since they are easier to dope so that the condition of the charge carriers at the normal operation temperature is fulfilled.
The spectral distribution of an illumination source (a lamp) used for such a switch is most often difficult to change, and the light with higher energies will in known switches be absorbed through the parts of the first layer located closest to the first side and generate free charge carriers (electron-hole pairs) there, whereas light of longer wave lengths will pass through the entire first layer without creating any free charge carriers. This means that the resistance of the parts of the first layer closest to the second side will be rather high resulting in a comparatively high on-state voltage and power losses in the closed state of the switch. Furthermore, space charge regions may be created, and the electric field will vary through the first layer and be much higher close to the second side, which will result in different problems, such as stability problems. These problems would also be present if an illumination source emitting light of only one determined wave length were used, since more light would be absorbed closer to the first side than near the second side.